Inasmuch as braking systems generally work by causing two components to rub against one another, it is normal for the components to gradually wear away. Therefore braking systems conventionally include wear indicators, so that the degree of wear can easily be monitored and so that when wear has reached a certain degree, the worn component can be replaced.
The prior art has developed many different approaches to wear indication in braking systems. For instance, U.S. Pat. No. 2,636,090 discloses a rod-and-piston brake shoe lining wear indicator. U.S. Pat. No. 3,952,295 discloses a brake shoe lining wear indicator that comprises a spring and trigger-release pin. U.S. Pat. No. 4,231,012 discloses a brake drum lining wear indicator comprising a plunger slidably mounted in a sleeve. U.S. Pat. No. 5,228,541, in its FIG. 4, shows a wear pin inserted in a hole in a brake pressure plate. The '541 patent claims a method of modifying a brake on an aircraft to accommodate a change in brake energy capacity. And U.S. Pat. No. 6,076,639 discloses a wear indicator for friction linings of brakes that comprises a displaceable measuring rod with one end in contact with the brake means and with the other end protruding outside the brake housing.
Wear pins are currently attached to brake pressure plates using a metal rivet which employs a through-hole in the pressure plate as an attachment point. The presence of the through-hole creates uneven wear patterns on the pressure plate and on the mating rotor. Also, the metal rivet is exposed to very high friction surface temperatures and is subject to rotation, both of which can lead to pull-out failure of the attachment. Wear pins made of steel are relatively heavy, and thus place a high radial load on the hole and/or retainer system used to position them.
In braking system parlance, a pressure plate is an annular carbon-carbon composite stator disc that transmits a force from the hydraulic system to the brake stack via the pistons. The brake stack, commonly referred to as a heatsink, comprises annular carbon-carbon discs of alternating stator and rotor elements. Pressure plates are often made of carbon-carbon composite material, while wear pins are typically made of steel. Conventionally, a wear pin is attached to a pressure plate on the non-friction back surface of the brake pressure plate and protrudes through a hole in the piston housing. Wear pin protrusion serves as a gauge of remaining wearable thickness of the brake stack during use.
FIGS. 1 and 2 of the present application illustrate conventional wear pin retainer designs. Both FIG. 1 and FIG. 2 are cross-sectional side views through a braking system pressure plate at a plane that includes the wear pin.
In FIG. 1, reference numeral 11 designates the wear pin. Wear pin 11 is held in place in wear pin housing 16 in pressure plate 12 by retaining clip 15. Wear pin 11 passes through piston housing 14. Pressure plate 12 has a friction surface 13 on the side thereof away from piston housing 14. Thus the wear pin retainer design shown in FIG. 1 uses only the wear pin and a retaining pin, with the wear pin mounted through the pressure plate.
In FIG. 2, reference numeral 21 designates the wear pin. Wear pin 21 is held in place in wear pin housing 26 in pressure plate 22 by steel cup 29. Steel cup 29 is attached to pressure plate 22 by rivet 27. Steel cup 29 has holes 28 for retaining clips (not shown). Wear pin 21 passes through piston housing 24. Pressure plate 22 has a friction surface 23 on the side thereof away from piston housing 24. As can be seen, then, the wear pin retainer design illustrated in FIG. 2 uses a steel cup which is riveted to the pressure plate. The wear pin is retained in the cup by means of retaining clips.
It should be noted that both of the conventional wear pin retainer designs require an opening on the friction surface of the pressure plate. This is undesirable for a variety of reasons, inasmuch at it may cause problems with respect to friction, wear, strength, and oxidation. Also, conventional designs are sometimes subject to loss of wear pins due to pin pull through.